LTE-A system introduces Carrier Aggregation (CA), which enables UE (User Equipment) to work on several carriers simultaneously. But in LTE system where UE works on single carrier, if inter-frequency handover or re-establishment occurs to UE, UE needs to switch the measurement configuration of source cell and target cell after handover or re-establishment success.
Therein, the present technologies involved are:
(1) Multi-Carrier System
In LTE and previous radio communication system, there is only one carrier in a cell, and the maximum bandwidth in LTE system refers to 20 MHz, as shown in FIG. 1.
In LTE Advanced (LTE-A) system, the peak rate is greatly improved compared with LTE, which is required to be 1 Gbps at downlink and 500 Mbps at uplink. Only using the carrier with maximum bandwidth of 20 MHz can hardly achieve the requirements of peak rate. Therefore, LTE-A system needs to expand the bandwidth available to terminal, thus the introduction of CA (Carrier Aggregation), that is, aggregate several continuous or discontinuous carriers under the same eNB (evolved Node-B,) and serve UE (User Equipment), so as to provide the required rate; thus these aggregated carriers are also called CC (Component Carrier). Each cell can be a cell, and those cells (CCs) under different eNB cannot be aggregated. To ensure UE of LTE can work under each aggregated carrier, each carrier cannot exceed 20 MHz at most. CA technology of LTE-A is shown in FIG. 2, where there are 4 carriers that can be aggregated under the eNB of LTE-A shown, and the eNB can perform data transmission on 4 carriers and UE simultaneously, to improve system throughput.
(2) LTE Measurement Mechanism
In LTE system, eNB sends measurement configuration information to UE through RRC (Radio Resource Control) signaling, and UE measures (intra-frequency, inter-frequency and inter-RAT) in accordance with the contents of measurement configuration information with the measurement results reported to the network.
RRC connection reconfiguration process is adopted by the network in measurement configuration, and the organization structure of measurement configuration information is:    a. Measurement object: Take the frequency as the basic unit, and each measurement object to be configured is considered as an independent frequency with separate measurement object ID.    b. Report configuration: It can be classified into event trigger report and periodicity trigger report according to trigger types, with separate ID possessed by each report configuration. Report configuration of event trigger type will allocate the threshold of a certain event and the time to trigger, while that of periodicity trigger type will allocate the purpose of periodicity trigger (e.g. report CGI).
Currently, there are five intra-frequency/inter-frequency measurement events in LTE system, including:    Event A1: Serving becomes better than threshold;    Event A2: Serving becomes worse than threshold;    Event A3: Neighbor becomes offset better than serving;    Event A4: Neighbor becomes better than threshold;    Event A5: Serving becomes worse than threshold1 and neighbor becomes better than threshold2.    c. Measurement ID: Independent ID can link a measurement object and report configuration simultaneously. In case of reaching the threshold of stating measurement, UE will judge whether to perform the measurement in accordance with the existence of measurement ID.    d. Other parameters may include measurement value configuration, measurement open threshold, speed state parameter and others.
There are three trigger modes for measurement report: event trigger report, periodicity report, event trigger periodicity report, all of which shall be distinguished according to the combination of various parameters in report configuration.
(3) Primary Component Carrier (PCC) and Secondary Component Carrier (SCC)
CA technology has introduced the concepts of Primary Component Carrier (PCC) and Secondary Component Carrier (SCC); UE has only one PCC (uplink and downlink) and others all refer to SCC. For measurement, PCC and SCC generally refer to frequency; serving cell on PCC is called Pcell while that on SCC is called Scell. PCC is characterized by:    1. Uplink PCC is used to transmit uplink control information of physical layer (ACK/NACK, CQI, D-SR, etc.);    2. Downlink PCC cannot be deactivated;    3. When radio link failure occurs to downlink PCC, UE will trigger RRC connection re-establishment process; (this phenomenon will not happen in case of radio link failure to SCC)    4. System information acquisition and update of downlink PCC is the same as LTE process;    5. NAS (Non-Access Stratum) information is acquired through downlink PCC.
Generally, the carrier which initial linked to UE is PCC and the follow-up configured and activated carriers refer to SCC. PCC has bound many features (e.g. uplink control information of physical layer is only transmitted on uplink PCC); therefore UE must possess PCC under CA working condition. All carriers (including PCC and SCC) configured to UE are called configured carrier assembly of UE, and eNB can activate/deactivate the carriers inside configured carrier assembly through MAC CE (MAC Control Element), only the activated carrier can perform data transceiving.
(4) Measurement in LTE-A CA System
Currently, there are some basic schemes for measurement of CA system: For each CC configured for UE (including the activated and deactivated), the network is capable of configuring A1 and A2 events for it (as shown in FIG. 4, cell 1 refers to Pcell, and cell 2 refers to Scell; frequency f1 refers to PCC and f2 refers to SCC; the network configures A1 and A2 events for cell 1 and cell 2);
For each CC configured for UE (including the activated and deactivated), the network is capable of configuring intra-frequency A3 event for its frequency; serving cell of the A3 event refers to the CC (Pcell or Scell) configured for UE on that frequency, while neighbor cell can be any other cell there (as shown in FIG. 4, the network configures intra-frequency A3 event measurement for f2 frequency and the serving cell refers to cell 2);
As for each frequency measurable for UE (including those configured to UE or not), the network can configure A3 event with serving cell of Pcell (A3-PCC) for it, that is, serving cell of A3-PCC refers to the current Pcell of UE, while neighbor cell can be any cell on other frequency or PCC (as shown in FIG. 4: for the unconfigured f3, the network configures A3-PCC measurement, serving cell refer to cell 1 and target cell is the cell on f3, including cell 3 and cell 7; for the configured f2, the network is configures A3-PCC measurement, serving cell refers to cell 1 and target cell is the cell on f2, including cell 2).
In course of performing the present invention, the inventor finds out there are at least the defects below in the prior art:
The prior art can only exchange measurement configuration specific to single carrier. For multi-carrier system, there is no solution yet as for how to perform multi-carrier handover of UE after inter-frequency exchange or re-establishment succeeds when UE works under CA state.